Electrochemical elements, such as non-aqueous electrolyte batteries and electric double layer capacitors, are used as back-up power sources for time-keeping functions, back-up power sources for semiconductor memories, auxiliary power sources for electronic devices, such as microcomputers and IC memories, batteries for solar powered watches, and power sources for driving a motor. Furthermore, in recent years, electrochemical elements have attracted attention as power sources for electrical automobiles and auxiliary systems for energy converting and storing systems.
The introduction of non-volatile memories for semiconductor memories and the reduction in the power consumption for elements having time-keeping functions make electrochemical elements having a large capacitance and large current less necessary. Rather, it is strongly desired to make it possible to mount electrochemical elements on a substrate by reducing the thickness of the element and developing reflow soldering.
Therefore, electrochemical elements in round form, such as coin and button forms, and electrochemical elements in square form have been proposed as described in Japanese Unexamined Patent Publication No. 2001-216952 and Japanese Unexamined Patent Publication No. 2004-356009. When the electrochemical element is in square form, the element cannot be sealed by crimping the container in square box form, unlike the electrochemical element in round form. Thus, a sealing plate is resistance welded to the upper portion of the container in square box form. In particular, an electrode pair made up of a positive electrode and a negative electrode, a separator, an electrolyte and the like are contained inside the square-box container, the sealing plate is placed on the upper portion thereof, and seam welding using resistance welding is carried out.
When seam welding using resistance welding is carried out, an electrode active material used for a negative electrode is pasted to the sealing plate in advance so that the negative electrode pasted to the sealing plate is compressed into the container from the top, and welding is carried out in a state where the sealing plate is pressed against the container. Therefore, there is a pasting process for pasting an electrode active material to the sealing plate before welding. When pasting between the electrode active material and the sealing plate is incomplete, the electrode active material is peeled from the plate, which may become the cause for malfunction, such as an increase in the internal resistance. Therefore, the pasting process requires a long period of time and a high level technology in order to secure the reliable pasting between the sealing plate and the electrode active material.
In addition, as electronic devices have been recently miniaturized, electrochemical elements have been required to be reduced in size. It is becoming more and more difficult to paste an electrode active material to a sealing plate which is small in size, which has become a problem in increasing the production efficiency and the reliability of the products.